• Dan Shi, Guo-Qiu Dong, Ke-Ren Shen, Yao Pan, Shu-Mei Wei, Ying Chen, and Ri-Sheng Yu.
• Department of Radiology (Shi D, Dong GQ, Shen KR, Pan Y, Chen Y, Yu RS), Department of Pathology (Wei SM), The Second Affiliated Hospital, Zhejiang University School of Medicine, Jiefang Road 88#, Hangzhou 310009, China.
• Medicine (Baltimore). 2021 Jan 15; 100 (2): e24054e24054.

RationalePrimary neuroendocrine tumors (NETs) of the retroperitoneum are extremely rare. The purpose of this case report is to highlight the unusual growth pattern and radiologic features of primary retroperitoneal NETs.Patient ConcernsA 46-year-old woman was found to have a retroperitoneal cystic and solid mass during a physical checkup.DiagnosesThe mass was mainly multiseptated in the cystic portion and had a bead-like, lobulated appearance. The solid portion showed restricted diffusion on diffusion-weighted imaging and obvious homogeneous enhancement. The cystic portion showed ring-like and septal enhancement. The patient was diagnosed with a grade 2 (G2) NET of the retroperitoneum after surgery.InterventionsThe patient underwent resection of the large retroperitoneal tumor.OutcomesThe patient returned 20 months later with tumor recurrence in the retroperitoneum. She was enrolled in a clinical trial for sulfatinib, and the mass was considerably reduced in size after 4 months. During a nearly 1.5-year follow-up, the mass gradually became slightly enlarged. The expression of somatostatin receptor 2 (SSTR2) was detected, and somatuline was administered as the current treatment.Lessons SubsectionsWhen a retroperitoneal mass presents as a well-defined cystic or solid hypervascular mass with a fibrous capsule, a primary retroperitoneal NET should be considered in the differential diagnosis.Copyright © 2021 the Author(s). Published by Wolters Kluwer Health, Inc.

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